Talk to any physician who treats allergies, and they'll liken their job to that of a police detective. Constantly on the hunt for the unknown offender, an allergy consult often seems more like a witness interrogation featuring a litany of probing questions.

Sometimes, the case is tough to crack, because you can be allergic to just about anything. Some people are even allergic to medications that are used to treat allergies, such as corticosteroids.

Still, seldom are these incidents one-of-a-kind phenomena. Even the most unexpected allergic response is likely to be duplicated in another person, somewhere.

In order to make the job easier for fellow allergy investigators, doctors will publish accounts of rare allergic reactions in medical journals and share their findings at medical conferences and on the Internet.

We've collected 11 of the most unusual allergy stories.

Water

Imagine being allergic to a substance that makes up about 70 percent of the earth and almost as much of our bodies. But for some, a rare allergy to water is harsh reality.

Michaela Dutton, 21, has aquagenic urticaria, which causes her to get hives when her skin comes in contact with water. While physical urticarias are not uncommon -- people can develop hives within minutes in response to ordinary stimuli including heat, cold and pressure -- sensitivity to water is far less common.

Dutton said she broke out in a red rash and white blisters after she took a bath about a week after her son was born three years ago. Although she ignored the reactions at first, her symptoms worsened and she went to see a doctor and a dermatologist who told her she had a water allergy.

"Water induced urticaria is very unusual -- there are not many cases ever reported," said Dr. Thomas Casale, chief of allergy and immunology at Creighton University and executive vice president of the American Academy of Allergy, Asthma & Immunology. "The mechanism has not really been defined."

It is certain, however, that people with aquagenic urticaria produce histamine from mast cells in the skin which causes redness, rashes and hives if they touch water. Dutton's sensitivity is such that she can only bathe for about 10 seconds each week and cannot drink water, juice, tea or coffee, opting for diet cola instead. She is also restricted from eating certain fruits and vegetables.

"It's not a problem with water in the body. It's when [water] is applied on top of the body," Casale said, citing additives as a possible cause for the allergic reaction seen on the skin.

Dutton, who lives in Walsall in the UK, also must be careful when holding her 3-year-old son. Her allergy was triggered after his birth and even his tears can cause hives.

"He doesn't really understand," Dutton said. "If he falls asleep I have to watch he doesn't dribble on me."

Physical urticarias tend to occur in individuals starting in their 20s and 30s but it is impossible to predict how long the condition will last.

"Some of these can be time limited but some can last for quite a few years," Casale said.

Most urticarias are treated with antihistamines but Casale said that because they are so rare, there have been no major breakthroughs in treatments.

Hot and Cold

About one in five people will develop hives at some point in their lives, but a much smaller number will get physical urticarias (urticaria is another word for hives) when they contact or experience some very ordinary things.

"If you put an ice cube on somebody that has cold urticaria, they're going to have a big welt right where the ice cube was," said Casale. And it's not just the cold. "It could be heat, could be sunlight. It could be vibration. It could be pressure."

In fact, some cases of heat-induced hives might be confused with exercise-induced anaphylaxis because exercise can raise body temperature.

Casale found the two could be distinguished for diagnosis by either heating the patient using a warming blanket or placing the patient's hand or leg in hot water. Only those with heat urticaria will develop hives.

An even smaller number of people with the condition may be diagnosed with aquagenic urticaria, meaning they are allergic to water.

"I actually saw a patient probably six or seven years ago. We put water on her and, boy, she just broke out in hives where the water hit her," said Casale, although he thought the reaction might have been provoked more so by the temperature of the water than the water itself.

There's no treatment for physical urticaria and "we still don't have a really good clue as to how [the hives] come about," said Casale. "These syndromes aren't extremely common and they're very difficult to study."

Cell Phones

Normally we're pretty careful about the kinds of things we let touch our faces, but how often has that concern come to mind when that thing is your cell phone?

In recent years, dermatologists have begun to see an increasing number of contact dermatitis patients who are allergic to their cell phones, or more specifically the nickel in their cell phones.

"Some people are extremely nickel-sensitive," said Dr. Lionel Bercovitch, a professor of dermatology at Brown Medical School.

Nickel is a metal that's used in a wide variety of products, including jewelry, belt buckles and watch bands. It's the most common cause of contact dermatitis in the developed world.

The symptoms of a nickel reaction range from redness to a more obvious rash, even blisters. Bercovitch suspects some cases involving cell phones are not being reported because the symptoms are being confused with facial eczema.

"My guess is that it's probably more common than we think, but it's just not widely recognized," he said.

Not all cell phones contain nickel. In an attempt to get an idea of how many phones might have the metal, Bercovitch tested 22 models and published the results in the Canadian Medical Association Journal in January 2008.

Ten devices were positive for the metal. In some, the nickel showed up around the menu buttons. In others, it appeared near the decorative logos, around the edge of the screen or on a part of the handset where paint was chipped.

"I think you need the perfect storm of somebody who's allergic to nickel in the first place, and somebody who uses the cell phone a lot, and a cell phone that has an exposed nickel area," said Bercovitch.

Chairs, Flutes and Other Items With Nickel

"We're starting to see people who have rashes in other places because of the unexpected content of nickel in those items," said Dr. Sharon Jacob, assistant professor of medicine and pediatrics at the University
of California at San Diego.

Jacob notes she's cared for children who've developed rashes on the back of their legs due to contact with nickel-plated studs on the seats of their classroom chairs.

"You can get blisters, you can get scarring," said Jacob. "We're seeing children as young as 4, and they're becoming scarred from this."

Other patients have more challenging cases to solve. Jacob once treated a girl who developed severe lip dermatitis after playing a flute containing the metal.

One problem with diagnosing the allergy is the rash could come a week after contact with nickel, so not everyone associates the symptoms with the potentially problematic object, said Jacob.

The number of allergy patients testing positive for a nickel reaction is on the rise in the United States. Many clinicians have attributed the increase, especially in men, to a growing number of ear and other piercings.

About 19 percent of patients with allergic dermatitis are sensitized to nickel, reported the North American Contact Dermatitis Group in data from 2003-2004. The number of such patients was much smaller in 1985-1990 -- about 11 percent.

How does one become sensitized to nickel? As Jacob explains it, for some people the intimate contact between the skin and the nickel used in earrings and ear posts can do the trick.

"In general, the first time you're in contact with nickel, you don't get a rash," said Jacob. But your immune system remembers the metal and eventually, after more exposure, you reach an "elicitation threshold" when "your skin is primed to react."

Jacob says she wants the American Academy of Dermatology to help push for regulations to limit the amount of nickel in products with prolonged skin contact, just as was done in Europe in the early '90s.

Black Henna Tattoos

For thousands of years, people in North Africa, the Middle East and South Asia have decorated their bodies with henna, but in recent decades, adulterations in the dye used for these temporary tattoos have resulted in painful allergic reactions.

Henna is a normally reddish-brown or greenish-brown vegetable coloring derived from lawsone, an ingredient found in the leaves of the henna shrub. Normal henna rarely causes allergic contact dermatitis, but darker henna containing an added chemical also found in hair dye called p-phenylenediamine, or PPD, can instigate severe skin problems and additional allergies.

In some cases, the tattooed skin of those who are allergic to PPD will swell and blister, with the contours of the bumpy skin conforming to the shape of the original tattoo.

"When they add the black hair dye at significant concentrations … we're seeing people with blistering and scarring to it," said Jacob.

It's not unusual for people who react to black henna to also react to hair dye, sometimes as a result of the black henna encounter.

While PPD in small amounts is still allowed in hair dyes, it has been banned from skin products in the United States since 1938. Still, henna artists hoping to capitalize on the popularity of the black coloring can mix the dangerous paste on their own.

"When you see somebody with a stand that says 'henna,' ask them what color the stain is when the paste comes off the skin," explained Catherine Cartwright-Jones, a retired henna artist and webmaster for the Henna Page. If the stain on your skin is black, "you've got a
problem."

Allergic to Dad?

When we think of food allergies, we typically envision a reaction that results after the allergic person puts the problematic food in his or her mouth. But that's not always how it goes.

Dr. Clifford Bassett, an allergy specialist at the Long Island College Hospital and the American Academy of Allergy, Asthma and Immunology, notes that 40 percent of food allergy reactions come not from eating a food trigger but from touching or inhaling it.

Such was the case with one of Bassett's patients, a young girl who developed hives after she hugged and kissed her father when he returned home from work.

"My job is to be sort of an investigative reporter," explained Bassett. "One of the things that we look at is contacts, things we put on the body."

In this case, it wasn't even something the daughter had put on herself. It turned out that the child was allergic to the nut-derived oil in her father's shaving cream, said Bassett.

"That was an episode of a food allergen present in a cosmetic, basically," said Bassett, who added that the problem went away after dad switched to another type of shaving cream.

Chocolate

They were special dairy-free chocolate chips. They weren't supposed to trigger an allergic reaction in Stefania Paciocco's son Gabriele, who at 5 years old already had a laundry list of food allergies including milk, tree nuts, peanuts and eggs.

"I would make a [cookie] batter without eggs in it and I would add [the dairy-free] chocolate chips," said Paciocco, who lives in Plymouth, Mich. "Whenever he would eat anything with these chocolate chips in it, he would start to scratch his throat."

Paciocco suspected her son was allergic to chocolate, but the boy's doctor did not initially believe it.

"Even I was skeptical," said Dr. Harvey Leo, a pediatric allergist and immunologist at C.S. Mott Children's Hospital in Ann Arbor, Mich. "Chocolate allergies are really rare," he explained, adding that most reactions to chocolate either are not true allergic reactions or they come as a result of exposure to nuts or milk in the chocolate chunks.

However, after some persistence by Paciocco, a skin test and a food challenge showed Gabriele, indeed, had
the allergy.

"I was shocked because I love chocolate," said Paciocco. "And I felt bad. He can't have a chocolate bar?"

In the end, both parent and doctor agreed persistence paid off in confirming an unusual and potentially dangerous allergy.

"I don't think patients should be afraid to challenge their doctor," said Leo.

"Dr. Leo and I have a really good relationship, so I feel really comfortable with him," added Paciocco. "You should always listen to that intuition."

Exercise

It's the perfect excuse: I can't exercise because I'm allergic to it.

Sure, it sounds like a joke but a rare condition called exercise-induced anaphylaxis can result in an allergic reaction during exercise, and it can be severe enough to kill the exerciser.

"They have all of the manifestations one would have if you had an acute allergic reaction to peanuts," explained Dr. Thomas Casale, chief of allergy and immunology at Creighton University and executive vice president of the American Academy of Allergy, Asthma & Immunology.

Most often reported with running or jogging, the exercise-allergic person might get hives, swelling, trouble breathing, low blood pressure, itching, nausea, a headache or wheezing.

Because some of the symptoms occur commonly during normal exercise, some people with exercise-induced anaphylaxis might not realize they have the problem.

In addition, for some people, the reaction only comes when exercise is combined with a certain food. Casale
noted several early reports of exercise-induced anaphylaxis from people who ate celery before they exercised.

"They could exercise, they were fine. They could eat celery, they were fine. They eat celery, then they exercise -- then they have an anaphylactic reaction," said Casale, who studied the phenomenon in the 1980s.

As is typical with most allergic reactions, the symptoms of exercise-induced anaphylaxis result from chemicals -- including histamines -- that are released by mast cells in the body. But Casale said researchers have yet to figure out why exercise, or the food/exercise combination, triggers the mast cells to act.

Still, doctors say there are ways for the afflicted to exercise and stay safe. You can exercise with a buddy, carrying adrenaline, and in the case of those with food triggers, avoid meals for two to four hours before and after exercise, Casale suggested.

Hairy Caterpillars

The pine processionary caterpillar and its cousin, the oak processionary caterpillar, look harmless enough, but the hairs that cover their bodies contain a toxin that can cause a serious allergic reaction. Add to that the fact that those hairs can sail right off the caterpillar like dandelion seeds, and you have an airborne allergen not unlike pollen.

"I think most people, when they think of anaphylaxis or allergic reactions, they're thinking about insects like wasps and bees," said John Klotz, an entomologist at the University of California, Riverside. But these caterpillars exemplify the range of creatures out there that don't need to sting you to cause a reaction.

Klotz is publishing a review of reports on dozens of animals and insects, including the hairy caterpillars, that can cause a severe allergic response.

"I thought it would be a good way of heightening awareness of the problem," he said.

Deemed as forest pests in Europe, the processionary caterpillars -- named as such because they form long lines when heading into or out of their nests -- have been implicated in numerous individual and group incidents.

In June 2004, more than 40 people who were sitting under an infested oak tree in the region of Saarland in Southwest Germany were sickened by the caterpillars although only a few actually touched them.

"Someone could inhale them [the hairs] or ingest them, and in some cases they could penetrate the skin," said Klotz. Some children have been hospitalized after eating the creatures. "A lot of times they'll pick up something furry and curious like a caterpillar, and you know, sometimes ingest them, just out of curiosity."

Asian Ladybugs

You've probably heard of indoor allergens such as mold or dust mites, but what about the Asian ladybug? In
recent years researchers have pegged the beetles as a source of allergic symptoms including hay fever, coughing, wheezing and watery eyes.

Asian ladybugs, formally known as multicolored Asian lady beetles, were introduced to the U.S. as early as 1916 to control the aphid population, but the beetles don't like the cold, so they regularly swarm into houses when the temperature drops.

"These things come in in the winter, and you have real trouble," said Dr. Thomas Platts-Mills, professor of medicine, allergy and clinical immunology at the University of Virginia. "And they actually bite, occasionally."

Indoor clusters can number in the thousands. Beginning in the late 1980s, homeowners began to report large indoor swarms in Louisiana.

The beetles are now well-established along the East Coast, in parts of the Midwest, and the Southeast. They look similar to North American ladybugs but vary in color from yellow to brown and scarlet, with polka-dots optional.

Allergy experts such as Platts-Mills refer to the Asian ladybug as a "new seasonal indoor allergen."

The best advice for preventing an Asian ladybug infestation is to seal the cracks and crevices in your home that could let them in, said Donald Lewis, professor of entomology at Iowa State University.

Once they're inside, Lewis suggests: "You can spray them to kill them, but you might as well use your vacuum cleaner to suck them up."

Cancer Drug

Medications are designed to make us better, but on occasion, a drug sparks an allergic reaction that can kill.

In the case of cetuximab, also known as Erbitux, an intravenous drug given to people with colon cancer as well as head and neck cancer, an unusually high rate of allergic reactions in patients from certain parts of the U.S. puzzled doctors.

When researchers studied cetuximab in clinical trials, they reported serious allergic reactions in 3 percent of patients taking the medicine. But when the drug became widely available, a significantly higher number of patients in Tennessee and North Carolina -- more than 20 percent -- had severe allergic reactions when they began using the same drug.

Researchers studying the geographical differences eventually discovered that most of the people who had an allergic reaction to cetuximab also had a previously existing antibody in their blood that targeted a specific sugar on the cetuximab molecule, causing the immune response.

"The antibodies were there before they were treated, and they predicted what happened to them," said Platts-Mills, who co-authored the research paper.

So where did the antibody come from, and why do more people in Tennessee and North Carolina have it?

Platts-Mills theorizes that patients developed the antibody before they took the drug when their bodies staged "a very aggressive immune response" to a seed tick bite, but he has yet to determine exactly why the person makes this particular antibody after the bite.

"In the area where this thing is going on, tick bites are much more common than people realize," said Platts-Mills.

The ongoing research is being closely followed by other allergists intrigued by the findings.